The present invention relates generally to fast MR imaging and, more particularly, to MR imaging with the acquisition of coil sensitivity calibration data.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Increasingly, there is a desire to expedite MR imaging. Expeditiously executing an MR scan not only increases patient throughput but also reduces the likelihood of motion-induced artifacts in the reconstructed images. To this end, a number of imaging techniques have been developed to expedite the MR data acquisition process. For instance, Fast Spin Echo (FSE), Fast Gradient Recalled Echo (FGRE), Echo Planar Imaging (EPI), and spiral imaging are examples of a few imaging techniques that have been developed to expedite MR data acquisition. The aforementioned fast imaging techniques are similar in that data acquisition is expedited in a manner by which the imaging volume is excited and sampled. While such techniques alone have made substantial advances in scan time, parallel imaging techniques can further accelerate data acquisition. Parallel imaging utilizes an array of surface coils to increase the step size between phase-encoding lines, or equivalently by reducing the size of the field of view and the amount of data collected.
Parallel imaging techniques not only expedite data acquisition, but are particularly advantageous in reducing aliasing or wrapping that occurs in the phase-encoding direction when an imaging object extends outside a field-of-view (FOV). In particular, parallel imaging techniques remove or reduce the aliasing by using surface coil B1 fields (sensitivities), to define or determine an unaliased spin distribution. This unaliased spin distribution may then be used to reduce, negate, or offset the aliasing or wrapping that may occur from imaging an object that extends outside the FOV. Moreover, parallel imaging techniques are often preferred since they may be carried out in half the scan time of a conventional MR scan.
Two exemplary parallel imaging reconstruction techniques have been developed to reduce aliasing in a reconstructed image. SENSitivity Encoding (SENSE) and SiMultaneous Acquisition of Spatial Harmonics (SMASH) are two such imaging techniques that remove aliasing in the image and k-space domains, respectively. In addition, several variations of the SMASH technique have been developed including AUTOSMASH and GeneRalized Auto-Calibrating Partially Parallel Acquisition (GRAPPA).
As mentioned above, parallel imaging techniques reduce aliasing using surface coil B1 fields to find an unaliased spin distribution. Information regarding the surface coil B1 fields or sensitivities is typically acquired with an external calibration or a self-calibration technique. With an external calibration, a separate scan distinct from the reduced FOV SENSE or SMASH scan acquires data over the subject volume to be scanned with the SENSE or SMASH scan. The external calibration method typically uses a conventional 2D or 3D gradient echo pulse sequence that is usually relatively fast (a few seconds to a minute) with low spatial resolution (e.g. 32×32×32 matrix for a 48×48×48 cm FOV), and acquires data from the full FOV, i.e. an FOV that will fully contain the subject anatomy to be scanned.
The external calibration method, while reasonably effective, does have drawbacks. For instance, the external calibration method can give rise to artifacts in the reconstructed image if the subject moves between application of the calibration scan and the imaging data acquisition (SENSE or SMASH scan). This can be particularly problematic for axial planes with breathholding if the breathhold size between the two scans is different. Additionally, the external calibration technique requires operator involvement to prescribe the scan and acquire the data, as well as requires the operator to correctly choose the calibration scan volume and location. That is, scanning a calibration volume that is smaller than the imaging data volume, or mis-centering the calibration volume, will result in slices that cannot be unwrapped or have poor alias unwrapping. Further, if multiple scan stations are used for parallel imaging at each station, the calibration volume must be prescribed for each station thereby further increasing the burden imposed on the operator when prescribing the calibration scan volume and location.
With the self-calibration or auto-calibration technique, instead of increasing the step size between phase encoding lines over the entire k-space during the SENSE or SMASH scan, the normal step size is used for a small number of lines near the center of k-space, e.g. 32 lines. These Nyquist-sampled lines are used to determine or otherwise calculate an image giving a sensitivity map or, in the context of AUTO-SMASH and GRAPPA, directly restore missing k-space lines without the step of reconstructing a calibration image. Similar to the external calibration method, however, the self or auto-calibration technique also has several drawbacks. For instance, the FOV of the calibration data is necessarily the same as the FOV of the SENSE or SMASH scan. Since the SENSE or SMASH scan usually does not image the full FOV, in order to reduce scan time and increase spatial resolution, the resulting calibration data may have aliasing or wrapping in the phase-encoding direction. That is, aliasing in the calibration data results in incomplete unwrapping of aliasing in the SENSE or SMASH acquired data. Moreover, since SENSE or SMASH-prescribed scans deliberately provide very low signal from static tissue, for instance, vascular imaging with paramagnetic contrast agents such as gadolinium, the resulting calibration will, therefore, also have very low signal which may negatively affect the accuracy of the acquired sensitivity data. Further, in contrast to the benefits of parallel imaging, self-calibration increases scan time. When very high reduction factors are used, the relative increase in scan time can be substantial.
It would therefore be desirable to have a system and method of MR imaging with aliasing or wrapping artifact reduction with an overall reduction in scan time.